The rapid growth of radio communication systems such as cellular radio has compelled designers to search for ways in which system capacity can be increased without reducing communication quality. One way in which increased capacity can be provided is by increasing the efficiency in which the available cellular spectrum is used, e.g., by changing from analog to digital communication techniques. In North America, this change was implemented by transitioning from the analog "AMPS" system to a digital system "D-AMPS" which was standardized as IS-54B and later as IS-136. Other technological improvements, such as the implementation of Time Division Multiple Access instead of Frequency Division Multiple Access, have also increased system capacity. Even with the implementation of more spectrally efficient technologies, the need for more capacity in cellular communication systems continues to be a concern.
Another way in which the capacity of cellular communications system can be increased is to provide additional spectrum. For example, the FCC originally allocated two blocks of frequencies (i.e., 825-845 MHz (uplink) and 870-890 MHz (downlink)) for cellular service in the United States. In 1987, the FCC allocated an additional 5 MHz to each frequency block to increase capacity. Of course, this solution is limited since the usable frequency spectrum is finite and existing communication systems other than cellular already occupy portions of the usable spectrum.
Land Mobile Radio (LMR) systems are allocated frequency blocks, i.e., 806-824 MHz (uplink) and 851-869 (downlink), which are contiguous with those of the cellular band as shown in FIG. 1. In contrast to cellular radio systems, LMR systems are transmission trunked systems commonly used to provide radio communication service between individual radio units of a particular organization. For example, police departments use a version of LMR (commonly referred to as public service trunked (PST) systems) to communicate between patrol cars and a dispatcher at police headquarters. LMR systems have historically been implemented as independent sites covering a relatively large geographic area and serviced by one (or a few) transmitting base stations. Cellular systems on the other hand cover an even wider geographical area divided into many smaller "cells" each of which is serviced by its own transmitting base station. More recently, LMR multisite systems have been developed and implemented to expand geographical coverage as well in the LMR arena. At each LMR site, an LMR operator is allocated a portion of the LMR spectrum within which a fixed frequency pair is typically selected for use as a control channel while all of the other frequencies are used for traffic.
In 1994, the FCC announced that the frequency spectrums allocated for LMR, cellular, and personal communications systems (PCS) would be uniformly regulated. An operator can therefore now use frequencies within the combined bandwidth in any desired manner. Coupled with other regulatory changes, for example those which allow LMR spectrum to be licensed on a wide-area basis rather than a site-by-site basis, LMR frequencies may now be used for cellular communications. Use of the LMR frequency spectrum for cellular communications is referred to herein as "downbanded cellular (DBC)."
To implement DBC systems that are compatible with cellular systems, several challenges must first be addressed. For example, conventional LMR systems operating in the United States have 25 Khz channel widths, whereas cellular system operating in accordance with IS-54B have 30 KHz channel widths. One solution to this problem is provided in the above-referenced U.S. patent application Ser. No. 08/622,311, entitled "Downband Cellular Systems and Methods" where the conventional channelization of the LMR spectrum is redefined in a manner which provides significant advantages. More specifically, for every six originally specified, 25 KHz LMR channels, five new 30 KHz DBC channels are specified. In this way, complete compatibility with cellular systems, e.g., allowing roaming between cellular and DBC systems, is achieved.
As can be seen in FIG. 1, there is only a 2 MHz gap between the maximum cellular transmit frequency (849 MHz) and the minimum LMR receive frequency (851 MHz). This small frequency gap contrasts with the 20 MHz maximum transmit/minimum receive frequency gap normally used in DAMPS cellular phones. The 20 MHz frequency gap spacing is satisfactory given the ceramic duplexing filters used in cellular radios to maintain isolation between the transmitted and received signals. However, current filter materials technology (ceramic duplexers included) are unable to permit construction of a single LMR "plus" cellular ceramic duplexer filter that provides the ideal inband "flatness" and out-of-band rejection for such a narrow 2 MHz gap shown in FIG. 1.
Accordingly, it is an object of the present invention to provide a duplexing arrangement which permits increased frequency band coverage into what has traditionally been separate radio communication frequency bands.
It is a further object of the present invention to provide such a duplexing arrangement using transceiver hardware already present in the radio conventionally configured to transceive in just one frequency band, e.g., a cellular radio.
It is a further object of the present invention to use only a single duplexer in an extended-band radio transceiver.
It is a further object of the present invention to manufacture such an extended band radio inexpensively and without increasing the number of components or the size of components.
A further object of this invention is to adapt the duplexing circuitry of a cellular radio which is compatible with the IS-136 specification for cellular phones and to permit additional "downbanded" communications in land mobile radio frequency bands.
The present invention provides a radio transceiver having the capability to transceive information over first and second different sets of frequencies. Each set of frequencies includes a transmit frequency band and a receive frequency band. One example of the first and second sets of frequencies are the transmission/reception allocation for land mobile cellular radios. The radio transceiver includes a transmitter, receiver, and antenna. Duplexing circuitry is connected between the antenna and the transmitter and receiver. Depending upon the mode of operation of the radio transceiver for communications either in the first or in the second set of frequencies, switches are provided for variously configuring the antenna, duplexer circuitry, receiver, and transmitter.
In one preferred example embodiment of the present invention, only a single duplexer is employed that permits the radio to transceive over both the first and second sets of frequencies. Switches connect the one duplexer to the antenna of the radio and to the receiver and transmitter. To operate in the first set of transmit/receiver frequency bands, the switches are set to connect the duplexer between the antenna and both the transmitter and receiver. To transceive in the other set of transmit/receiver frequency bands, the switches are set to bypass the duplexer in connecting the antenna and the receiver at least in the receive path.
Thus, the present invention therefore may be used to adapt a cellular radio that originally transceives over formally designated cellular transmit and receive frequency bands to transceive in land mobile radio (LMR) transmit and receive frequency bands contiguous with the cellular transmit and receive frequency bands using only a single cellular duplexer. Cellular frequency transmissions and receptions are routed as usual through the duplexer. LMR frequency transmissions may be routed through the duplexer, while LMR receptions are routed through the switches.
In one example application to time slot based communications systems, e.g., TDMA systems, the control of the switches in the extended frequency bands is accomplished using a control signal generated based on time slot alignment timing. When transceiving in the cellular frequency bands, the switches are statically set. In the LMR extended bands, the switches are set dynamically. When the radio is transmitting, the antenna is connected to the transmitter through the duplexer during a transmit time slot. During the transmit time slot, the switches effectively isolate the antenna and the receiver. During a receive time slot, the antenna is connected to the receiver bypassing the one duplexer.
In a preferred embodiment, the switches are implemented using gallium arsenic (GaAs) field effect transistors (FETs) having low insertion loss and rapid switching speed. These switches maintain the required transmit-to-receive isolation that would otherwise be provided by a second duplexer included specifically for the extended frequency band.
Thus, the present invention provides radios with greater geographic coverage as well as improved system access in crowded areas (due to the additional channels provided in the extended frequency bands), and does so in one embodiment without adding a costly, second duplexer to handle calls in the extended frequency bands. Moreover, existing transceiver circuitry like that in conventional cellular radio telephones can be adapted for extended frequency band communications.